1. Field of the Invention
The invention relates to filaments used as electron emitting cathodes in electron impact ion sources for mass spectrometers (MS).
2. Description of the Related Art
Electron impact ionization, or more correctly Electron Ionization (EI), is a common type of ionization in gas chromatography-mass spectrometry (GC-MS). The EI source offers predictable fragmentation favorable for compound identification using commercially available libraries with several hundred thousand reference spectra, e.g., the library of the National Institute for Standards and Technology (NIST). The EI source furthermore offers uniform response for most compounds because the ionization efficiency is mostly not compound dependent.
The classical EI ion source is the cross-beam ion source wherein an electron beam generated by a linear glow cathode is accelerated through a slit to about 70 electronvolts, is guided by a weak magnetic field through an ionization region, exits through another slit and hits an electron detector used to regulate the electron current by controlling the electric current through the cathode. FIG. 1 shows schematically such a known cross-beam EI ion source. Effluents of the GC are blown through the ionizing electron curtain, and the ions generated are drawn out of the ionization region through slitted electrodes. This type of ion source is ideally suited for mass spectrometers operated with slits, e.g. magnetic sector mass spectrometers.
Today, however, most mass spectrometers are designed to accept cylindrically symmetric ion beams because they are regularly equipped with elongate quadrupole ion guides or quadrupole filters which encase a cylindrical inner volume. Ion sources with slits generating non-cylindrical ion beams no longer fulfill modern requirements in an optimum way. This mismatch may lead to ion beam losses in the ion source or in the ion extraction optics, or to an undesired widening of the ion energy distribution, or to an ion beam symmetry distortion further down the MS.
For a better match with the rest of the ion path into the mass spectrometer, cylindrically symmetric EI ion sources and especially cylindrically symmetric EI filament arrangements have been developed (see, e.g., M. DeKieviet et al., “Design and performance of a highly efficient mass spectrometer for molecular beams”, Rev. Scient. Instr. 71(5): 2015-2018, 2000, or A. V. Kalinin et al., “Ion Source with Longitudinal Ionization of a Molecular Beam by an Electron Beam in a Magnetic Field”, Instr. and Exp. Techn. 49(5): 709-713, 2006).
In the cited articles, ring-shaped filaments have been mounted in the stray field of the coil of an electromagnet so that the electrons are accelerated along the field lines into the center of the coil, thereby forming a narrow tubular electron beam. This principle is shown schematically in FIG. 2. The effluents of the GC are blown as a molecular beam through the ring-shaped filament into the coil of the magnet. The molecules of the effluents are ionized on the fly with high efficiency by the tubular electron beam.
A classical ring-shaped filament arrangement is shown in FIG. 3. Circular or cylindrically symmetric filament assemblies, such as ring-shaped filaments, however, run the risk of losing shape after cycles of repeated heating and cooling. Providing additional support posts used to reduce the freedom to deform, as shown in FIG. 4 for example, results in heat being carried away via the posts and leads to different electron emission characteristics over the regions of non-uniform temperature.
In view of the foregoing, there is a need for filament arrangements for EI sources in mass spectrometers, which do not lose shape and show an electron emission as constant as possible along the filament arrangement.